The overall goal of this proposal is to identify the mechanisms by which an aging systemic environment is unable to recruit muscle specific stem cells, termed satellite cells (SCs), to contribute to skeletal muscle hypertrophy. The Aims of this proposal are two-fold: 1.) to separate the intrinsic properties of the SC proper from the effects of the milieu in aging, and 2.) to test the role of GH, IGF-I, TNF-a, and IL-6 on SC function in the context of the age of the milieu. The approach of Aim 1 will utilize a crossover design, with young primary human SC cultures separately receiving media containing young or old human serum. Likewise, old SCs will separately receive media containing young or old serum. To account for the influence of mechanical load, each media application will occur with cells under both cyclically stretched and unstretched conditions using a FlexCell system. The rationale for this approach is that decrements in SC function with age have been noted, but it is unknown whether these decrements are due to the SC proper or an altered milieu. Further, the interaction of the age of the systemic environment and mechanical load on SC phenotype is unknown. Each treatment group will be evaluated for proliferation (via BrdU incorporation), apoptosis (via TUNEL staining), differentiation (via Western blotting of differentiation markers), and fusion (via immunohistochemical techniques) of SC, which are reflective of the four processes necessary for SC mediated hypertrophy. The overall approach of Aim 2 is to manipulate levels of GH, IGF-I, TNF-a, and IL-6 in both young and old serum to determine the influence of each over proliferation, apoptosis, differentiation, and fusion of in vitro SCs isolated from old subjects. In addition, each serum treatment group will be analyzed in the stretched and un-stretched conditions. The rationale for this approach stems from the fact that several candidate molecules have been previously identified that affect SC function when applied in isolation, however the effect of these specific factors in the context of an aged in vitro environment under mechanical load has not been determined. Overall, these studies will improve our understanding of muscle hypertrophy in aging and will direct future efforts to combat age-related loss of muscle mass and frailty. Relevance to Public Health: This work will seek to determine the mechanisms by which muscle from aged individuals is unable to grow and regenerate to the same extent as muscle from the young. Identification of the mechanisms by which this phenomenon occurs will direct future efforts aimed at therapy for age-related loss of muscle mass.